Image encoding/decoding method and device using color coordinate axis conversion

ABSTRACT

Disclosed is image encoding/decoding method using color coordinate axis conversion, comprising the steps of: acquiring pixel distribution information, on first color coordinate system, about an image; determining, on the basis of the pixel distribution information, first component having the widest variance from among the components of the first color coordinate system; acquiring an intermediate color coordinate system by rotating, around a starting point, a coordinate axes of the first color coordinate system so that variance of the first component is maximized; determining a second component having the widest variance from among components excluding the first component in the intermediate color coordinate system; acquiring a second color coordinate system by rotating, around the starting point, a coordinate axis of the intermediate color coordinate system so that variance of the second color component is maximized; and encoding the image on the basis of pixel distribution information on the second color coordinate system.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is U.S. National Phase application under 35U.S.C. § 371 of an International application No. PCT/KR2019/013933 filedon Oct. 23, 2019, which is based on and claims the benefit of conventionpriority to Korean Patent Application No. 10-2018-0132672, filed on Nov.1, 2018 in the Korean Intellectual Property Office, the disclosure ofwhich is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to a method and device for encoding anddecoding a picture using transformation of color coordinate axes and,more specifically, to a method and device for enhancing a compressionefficiency by transforming axes of a YCbCr color coordinate systemduring an encoding or decoding of a picture.

BACKGROUND

With recent development of communication technologies, transmissions ofpictures are rapidly increasing. However, a picture signal generally hasan enormous data size and needs to be compressed to reduce transmissioncosts. Accordingly, standards for compressing still pictures such asJoint Photographic Experts Group (JPEG) and standards for compressingmoving pictures such as Moving Picture Experts Group (MPEG) has beenestablished.

However, picture qualities and compression ratios are still at issueregardless of the compression standards, and researches have beenconducted to reduce the degradation of picture qualities and achievehigher compression ratio.

SUMMARY

Provided is a method of encoding and decoding a picture using atransformation of color coordinates axes.

Provided is a device for encoding and decoding a picture using thetransformation of color coordinates axes.

According to an aspect of an embodiment, a picture encoding methodincludes: acquiring pixel distribution information for a picture in afirst color coordinate system; determining a first color componentshowing a largest variance in pixel values among color components ofpixels in the first color coordinate system on the basis of the pixeldistribution information; rotating coordinate axes of the first colorcoordinate system around an origin such that the variance of the firstcolor component is maximized to acquire an intermediate color coordinatesystem; determining a second color component showing a larger variancein pixel values between color components, excluding the first colorcomponent, of the intermediate color coordinate system; rotatingcoordinate axes of the intermediate color coordinate system around theorigin such that the variance of the second color component is maximizedto acquire a second color coordinate system; and encoding the picture onthe basis of pixel distribution information in the second colorcoordinate system.

The method may further include: generating coordinate axestransformation information for the picture on the basis of a differencebetween the pixel distribution information in the first color coordinatesystem and the pixel distribution information in the second colorcoordinate system.

The method may further include generating additional information aboutthe picture on the basis of the coordinate axes transformationinformation for the picture.

The method may further include: determining whether to transformcoordinate axes for the picture on the basis of a difference between thepixel distribution information on the first color coordinate system andthe pixel distribution information on the second color coordinatesystem.

The method may further include: generating information of whether thecoordinate axes are transformed or not depending on whether thecoordinate axes are transformed or not; and generating additionalinformation about the picture based on the information of whether thecoordinate axes are transformed or not.

The operation of acquiring the pixel distribution information for thepicture in the first color coordinate system may include: acquiring thepixel distribution information for the picture in the first colorcoordinate system on the basis of pixel distribution information for atleast one another picture in the first color coordinate system.

The method may further include: generating additional information aboutthe picture on the basis of information about the at least one anotherpicture.

The first color coordinate system may include a YCbCr color coordinatesystem, and the first color component may include a Y-component.

According to another aspect of an embodiment, a picture encoding deviceincludes: a processor; and a memory storing at least one instruction tobe executed by the processor. When executed by the processor, the atleast one instruction causes the processor to: acquire pixeldistribution information for a picture in a first color coordinatesystem; determine a first color component showing a largest variance inpixel values among color components of pixels in the first colorcoordinate system on the basis of the pixel distribution information;rotate coordinate axes of the first coordinate system around an originsuch that the variance of the first color component is maximized toacquire an intermediate color coordinate system; determine a secondcolor component showing a larger variance in pixel values between colorcomponents, excluding the first color component, of the intermediatecolor coordinate system; rotate coordinate axes of the intermediatecolor coordinate system around the origin such that the variance of thesecond color component is maximized to acquire a second color coordinatesystem; and encode the picture on the basis of pixel distributioninformation in the second color coordinate system.

The at least one instruction may further include an instruction causingthe processor to: generate coordinate axes transformation informationfor the picture on the basis of a difference between the pixeldistribution information in the first color coordinate system and thepixel distribution information in the second color coordinate system.

The at least one instruction may further include an instruction causingthe processor to: generate additional information about the picture onthe basis of the coordinate axes transformation information for thepicture.

The at least one instruction may further include an instruction causingthe processor to: determine whether to transform coordinate axes for thepicture on the basis of a difference between the pixel distributioninformation on the first color coordinate system and the pixeldistribution information on the second color coordinate system.

The at least one instruction may further include an instruction causingthe processor to: generate information of whether the coordinate axesare transformed or not depending on whether the coordinate axes aretransformed or not; and generate additional information about thepicture based on the information of whether the coordinate axes aretransformed or not.

The at least one instruction may further include an instruction causingthe processor to: acquire the pixel distribution information for thepicture in the first color coordinate system on the basis of pixeldistribution information for at least one another picture in the firstcolor coordinate system.

The at least one instruction may further include an instruction causingthe processor to: generate additional information about the picture onthe basis of information about the at least one another picture.

The first color coordinate system may include a YCbCr color coordinatesystem, and the first component may include a Y-component.

The present disclosure enhances the compression efficiency when thepicture is encoded using a YCbCr color coordinate system such as in JPEGand MPEG compressions.

The present disclosure is applicable to various picture encoding methodssince the transformation of color coordinate axes are performed in earlystage of a picture encoding procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the disclosure may be well understood, there will now bedescribed various forms thereof, given by way of example, referencebeing made to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a picture encoding and decoding system;

FIG. 2 is a block diagram of a picture encoding device;

FIG. 3 is a block diagram of a picture decoding device;

FIG. 4 shows a sample picture;

FIG. 5 is a graph showing a pixel distribution of the sample picture inan RGB color space;

FIG. 6 is a graph showing a pixel distribution of the sample picture ina YCbCr color space;

FIG. 7 is graph showing a pixel distribution of the sample picture in acolor coordinate system obtained by a transformation of color coordinateaxes according to an embodiment of the present disclosure;

FIG. 8 is a graph showing an enhanced compression efficiency when apicture encoding is performed after a transformation of color coordinateaxes according to an embodiment of the present disclosure;

FIG. 9 is a block diagram of a picture encoding device according to anembodiment of the present disclosure; and

FIG. 10 is a flowchart showing a picture encoding method according to anembodiment of the present disclosure.

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

While the present disclosure is susceptible to various modifications andalternative embodiments, specific embodiments thereof are shown by wayof example in the accompanying drawings and will be described in detail.However, it should be understood that there is no intention to limit thepresent disclosure to the particular forms disclosed, but rather thepresent disclosure is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the presentdisclosure. Like numbers refer to like elements throughout thedescription of the drawings.

It will be understood that, although the terms first, second, A, B, etc.may be used herein to describe various elements, the elements should notbe limited to the terms. The terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any one or combination of theassociated listed items.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to another element or be connected or coupled toanother element with still another element disposed therebetween. Incontrast, when an element is referred to as being “directly connected”or “directly coupled” to another element, there are no interveningelements present.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting to the presentdisclosure. As used herein, the singular forms “a,” “an,” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises,” “comprising,” “includes,” and/or “including,” when usedherein, specify the presence of stated features, integers, steps,operations, elements, and/or components but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components, and/or groups thereof.

Unless otherwise defined, all terms including technical and scientificterms used herein have the same meaning as commonly understood by one ofordinary skill in the art to which this disclosure belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

Typically, a video may include a series of still pictures, and the stillpictures may be divided in units of groups of pictures (GOPs), and eachstill picture may be referred to as a picture or frame. As an upperconcept, a GOP, a sequence, and other units may exist, and each picturemay be divided into predetermined areas such as slices, tiles, blocks,and the like. In addition, a single GOP may be grouped in units of Ipictures, P pictures, and B pictures. The I picture may refer to apicture that is encoded/decoded by itself without using a referencepicture, and the P picture and the B picture refer to picturesencoded/decoded by performing processes, such as motion estimation,motion compensation, and the like, using a reference picture. Ingeneral, in the case of a P picture, an I picture and a P picture may beused as a reference picture, and in the case of a B picture, an Ipicture and a P picture may be used as a reference picture, but theabove described definition may be changed by a setting ofencoding/decoding.

Here, a picture referenced for encoding/decoding is referred to as areference picture, and a block or pixel referenced for encoding/decodingis referred to as a reference block or a reference pixel. In addition,data referenced for encoding/decoding may include not only pixel valuesof a spatial domain, but also coefficient values of a frequency domain,and various pieces of encoding/decoding information generated anddetermined during encoding/decoding processes.

The minimum unit constituting a picture may be a pixel, and the numberof bits used to represent one pixel is referred to as a bit depth. Ingeneral, the bit depth may be 8 bits, and different bit depths may besupported according to encoding settings. As for the bit depth, at leastone bit depth may be supported according to a color space. In addition,a picture may be constructed as at least one color space according to acolor format of a picture. A picture may be constructed as one or morepictures having a predetermined size or one or more pictures havingdifferent sizes according to the color formats. For example, in the caseof YCbCr 4:2:0, a picture may be composed of one luminance component (Yin the present example) and two color difference components (Cb/Cr inthe present example), in which the color difference component and theluminance components may be composed in a horizontal length-verticallength composition ratio of 1:2. As another example, in the case ofYCbCr 4:4:4, the color difference component and the luminance componentsmay be composed in a composition ratio of a horizontal length and avertical width that are same as each other. As in the above example,when one or more color spaces are constructed, a picture may bepartitioned into the respective color spaces.

In the present disclosure, descriptions will be made based on a partialcolor space (Y in the present example) according to a partial colorformat (YCbCr in the present example), and the remaining color spaces(in the present example, Cb and Cr) according to the color format may besubject to the same or a similar application (a setting dependent on aspecific color space). However, each color space may also have apartially different applications (a setting independent of a specificcolor space). In other words, the setting dependent on each color spacemay refer to a setting proportional to or dependent on the compositionratio of respective components (for example, 4:2:0, 4:2:2, or 4:4:4),and the setting independent of each color space may refer to a settingindependent of the composition ratio of respective components, or asetting independently given only for the corresponding color space. Inthe present disclosure, depending on the encoder/decoder, someconfigurations may have an independent setting or a dependent setting.

Setting information or syntax elements required in the picture encodingprocess may be determined at a unit level of video, sequence, picture,slice, tile, block, and the like and may be included in a bit stream inunits of video parameter sets (VPSs), sequence parameter sets (SPSs),picture parameter sets (PPSs), slice headers, tile headers, or blockheaders and transmitted to the decoder. The decoder may perform parsingin units of the same level as that in the encoder to reconstruct thesetting information transmitted by the encoder and use the settinginformation in the picture decoding process. Each parameter set has anidentifier (ID) value, and a lower parameter set may have an ID valuefor referring to an upper parameter set is referenced. For example, alower parameter set may refer to information about an upper parameterset having an ID value that matches that of the lower parameter amongone or more upper parameter sets. In the above described examples ofvarious units, a certain unit including one or more other units may bereferred to as an upper unit, and the included units may be referred toas lower units.

In the case of setting information generated from the upper unit, eachunit may include information about an independent setting or may includecontent about a setting dependent on a previous, subsequent, upper unit,or the like. Here, the dependent setting may be understood as settinginformation of the corresponding unit that is represented as flaginformation indicating that the unit follows the setting of theprevious, subsequent, and upper unit (e.g., following with a 1-bit flagof 1 and not following with a 1-bit flag of 0). The setting informationin the present disclosure will be described based on an example of anindependent setting, but the present disclosure may include an exampleof addition or substitution about a dependence on the settinginformation of the previous, subsequent, or upper unit of the currentunit.

In the encoding/decoding of a video, in general, the encoding/decodingmay be performed according to an input size, but encoding/decoding mayalso occur through size adjustment. For example, in a hierarchicalencoding scheme (scalable video coding) to support scalability ofspatial, temporal, and picture quality, adjustment of overallresolution, such as expansion and reduction of a picture may exist, orexpansion and reduction of a partial picture may exist. Thecorresponding information may be switched by allocating selectioninformation in the units, such as a VPS, a SPS, a PPS, and a SliceHeader described above. In this case, the upper and lower relationshipbetween the units may be set in the order of VPS, SPS, PPS, and SliceHeader.

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of a picture encoding and decoding system.

Referring to FIG. 1 , a picture encoding device 105 and a picturedecoding device 100 may be a user terminal such as a personal computer(PC), a notebook computer, a personal digital assistant (PDA), aportable multimedia player (PMP), a PlayStation Portable (PSP), awireless communications terminal, a smart phone, or a television (TV),or a server such as an application server and a service server. Also,the picture encoding device 105 and the picture decoding device 100 maybe one of various apparatuses which includes a communication device suchas a communication modem configured to communicate with other devicesthrough wired or wireless communication networks, a memory 120 or 125configured to store programs and data for performing an inter frameprediction or an intra frame prediction to encode or decode the picture,and a processor 110 or 115 configured to perform operations and controlsby executing the programs.

A picture encoded into a bitstream by the picture encoding device 105 istransmitted to the picture decoding device 100 in real time or non-realtime through a wired or wireless communications network such asInternet, a short-range wireless communications network, a wirelesslocal area network (LAN), a WiBro network, and a mobile communicationsnetwork, or through a communication interface such as a cable oruniversal serial bus (USB). The picture bitstream may be decoded by thepicture decoding device 100 to be reconstructed and reproduced as theoriginal picture. Also, the picture encoded into the bitstream by thepicture encoding device 105 may be transferred from the picture encodingdevice 105 to the picture decoding device 100 through acomputer-readable recording medium.

The picture encoding device and the picture decoding device describedabove may be separate devices or may be integrated into a single pictureencoding and decoding device. In a latter case, some components of thepicture encoding device may overlap with some components of the picturedecoding device, and thus the picture encoding and decoding device mayinclude duplicate components or may be implemented such that a sharedcomponent may perform common functions.

Accordingly, in the following descriptions for configurations andoperations of embodiments, repetitive description of the overlappingcomponents and operation principles thereof will be omitted forsimplicity. Further, since the picture decoding device may be acomputing device that performs reverse operations of the pictureencoding process performed by the picture encoding device, the followingdescription will be focused on the picture encoding device.

The computing device may include a memory storing a program or asoftware module for implementing a picture encoding method and/or apicture decoding method, and a processor connected to the memory toexecute the program. Here, the picture encoding device may beabbreviated as a picture encoder and the picture decoding device may beabbreviated as a picture decoder.

FIG. 2 is a block diagram of the picture encoding device.

Referring to FIG. 2 , a picture encoding device 20 may include apredictor 200, a subtractor 205, a transformer 210, a quantizer 215, aninverse quantizer 220, an inverse transformer 225, an adder 230, afilter 235, an encoded picture buffer 240, and an entropy encoder 245.

The predictor 200, which may be implemented using a prediction softwaremodule, may form a prediction block for a block to be encoded byperforming the intra frame prediction or the inter frame prediction forthe block. The predictor 200 may form the prediction block by predictingthe current block to be encoded at the present time. In other words, thepredictor 200 may predict pixel values of the pixels in the currentblock to be encoded by the intra frame prediction or the inter frameprediction to form the prediction block in which the pixels have thepredicted pixel values. The predictor 200 may output informationrequired for forming the prediction block such as information about aprediction mode, i.e., the intra frame prediction or the inter frameprediction, so that such information may be encoded with the pixelvalues.

The subtractor 205 may subtract the prediction block from the currentblock to form a residual block. In other words, the subtractor 205 mayform the residual block containing a residual signal in a block unit bycalculating a difference between a pixel value of each pixel in thecurrent block and a pixel value of each pixel in the prediction block.Also, the subtractor 205 may generate the residual block according to aunit other than the block that is obtained by a partitioning of theblock, which will be described below.

The transformer 210 may transform the residual block into a frequencydomain to transform pixel values of the residual block into transformcoefficients. Here, the transformer 210 may transform the residualsignal into the frequency domain using at least one of various transformschemes that transform a spatial domain picture signal into a frequencydomain picture signal such as Hadamard transform, discrete cosinetransform (DCT)-based transform, discrete sine transform (DST)-basedtransform, and Karuneen Lube transform (KLT)-based transform. Thetransformation of spatial domain pixel values yields the transformcoefficients.

A size or shape of a transform block may be determined through a blockpartitioning which will be described below. A square or rectangularblock may formed according to the block partitioning. The blockpartitioning operation may be affected by transform-related settings,supported by the encoding and decoding device, such as the size or shapeof the transform block.

The size and shape of each transform block may be determined accordingto a cost for encoding each candidate size and candidate shape for thetransform block. The picture data of each transform block may be encodedalong with partitioning information such as the size and shape of thetransform block.

The transform may be performed by using a one-dimensional transformmatrix. Each transform matrix may be used adaptively in a horizontal orvertical direction. The adaptive use of the transform matrix may bedetermined by various factors such as the size of the block, the shapeof the block, the type of the block (e.g., luminance and chromadifference), an encoding mode, the prediction mode, a quantizationparameter, and encoding information for neighboring blocks, but thepresent disclosure is not limited thereto.

For example, in case that the prediction mode is a horizontal mode inthe intra frame prediction, a DCT-based transform matrix may be used inthe vertical direction while a DST-based transform matrix may be used inthe horizontal direction. On the other hand, in case that the predictionmode is a vertical mode in the intra frame prediction, the DCT-basedtransform matrix may be used in the horizontal direction while theDST-based transform matrix may be used in the vertical direction.However, the transform matrix is not limited thereto.

Transform-related information may be determined according to one or morefactors among the size of the block, the shape of the block, theencoding mode, the prediction mode, the quantization parameter, and theencoding information for the neighboring blocks, and may be signaledimplicitly or explicitly. The transform-related information may betransmitted in a unit of a sequence, a picture, a slice, or a block.

For an example of an explicit signaling of the transform-relatedinformation, when there are two or more transform matrices forhorizontal and vertical directions as a group of candidates, thetransform-related information signaled to a decoder may includeinformation about transform matrix having been used for each direction.Also, the transform-related information signaled to the decoder mayinclude information of a pair of transform matrices having been used forthe horizontal and vertical directions selected from a group ofcandidates consisting of a plurality of pairs.

Meanwhile, transforms may be omitted partially or entirely in dependingon the characteristics of the picture. For example, transforms of one orboth of the horizontal and vertical components may be omitted when theintra frame prediction or the inter frame prediction is not properlyperformed and the difference between the current block and theprediction block is large since an encoding loss may be large due to anincrease in the prediction residuals in such a case. The omission may bedetermined according to one or more factors among the encoding mode, theprediction mode, the size of the block, the shape of the block, the typeof the block (e.g., luminance and chroma difference), the quantizationparameter, and the encoding information for the neighboring blocks. Theomission of transforms may be signaled implicitly or explicitly and maybe transmitted in a unit of the sequence, the picture, and the slice.

The transformer 210 may provide information required to perform thetransform process (e.g., setting information such as a size of thetransform, a form of the transform, a type of the transform, a number oftimes of the transforms, and whether the transform was applied) to theentropy encoder 245, so that the entropy encoder 245 encodes the suchinformation.

The quantizer 215 may quantize the transform coefficients for theresidual block. The quantizer 215 may quantize the transformcoefficients for the transformed residual block by a dead zone plusuniform threshold quantization, a quantization using a weighted matrix,an improved quantization scheme thereof, or the like. The quantizer 215may set one or more of the quantization schemes as candidates and selectone according to the encoding mode or the prediction mode. Thequantization parameter may be determined in a unit of a block. Also, thequantization parameter may be determined in a unit of the sequence, thepicture, the slice, or blocks.

In an embodiment, the quantizer 215 may predict a current quantizationparameter using one or more quantization parameters derived fromneighboring blocks such as left, upper left, upper, upper right, andlower left blocks of the current block. Here, when there happens achange or addition of available neighboring blocks such as right andlower right blocks due to a change in an encoding order of or the like,the quantizer 215 may predict the current quantization parameterreflecting such a change.

When there exists no quantization parameter predicted from theneighboring blocks, i.e., the current block is located at a boundary ofthe picture or the slice, or the like, the quantizer 215 may output ortransmit a difference from a basic parameter transmitted in a unit ofthe sequence, the picture, or the slice. However, when there is aquantization parameter predicted from the neighboring blocks, thequantizer 215 may transmit a difference calculated using thequantization parameter of the corresponding block.

The quantizer 215 may have a quantization weighted matrix to apply to aninter frame coding unit or an intra frame coding unit. Also, thequantizer 215 may apply a different weighted matrix according to theinter frame prediction mode. The weighted matrix may have a size of M×N.In case that the quantization block size is the same as the block size,the quantization coefficients may be set to be different for eachposition of the frequency components. Also, the quantizer 215 may employone of the various existing quantization methods, and a samequantization method may be employed in the encoding device and thedecoding device. Information about the quantization method may besignaled in a unit of the sequence, the picture, or the slice.

The quantizer 215 may transfer information required for performing thequantization process (e.g., the setting information such as thequantization parameter, the quantization type, whether the quantizationwas applied, and a quantization range, and the quantization matrix) tothe entropy encoder 245, so that the entropy encoder 245 encodes thesetting information.

The inverse quantizer 220 inversely quantizes the quantized coefficientsquantized by the quantizer 215 for the residual block. That is, theinverse quantizer 220 inversely quantizes the quantized transformcoefficients to generate frequency domain coefficients for the residualblock.

The inverse transformer 225 inversely transforms the transformcoefficients output by the inverse quantizer 220 for the residual block.In other words, the inverse transformer 225 inversely transforms thetransform coefficients for the residual block to reconstruct spatialdomain pixel values for the residual block. The inverse transformer 225may perform the inverse transformation through an inverse process of thetransform process used by the transformer 210.

The adder 230 reconstructs the current block by adding the predictionblock predicted by the predictor 200 and the residual blockreconstructed by the inverse transformer 225. The reconstructed currentblock may be stored in the encoded picture buffer 240 as a referencepicture or a reference block and may be used as a reference when a nextblock following the current block, another block, or another picture isencoded in the future.

The filter 235 may performs one or more post-filtering processes such asa deblocking filtering, a sample adaptive offset (SAO), and an adaptiveloop filter (ALF). The deblocking filter may remove a block distortionoccurring at a boundary between blocks in a reconstructed picture. TheALF may perform the filtering based on a value obtained by comparing apicture reconstructed by filtering the block distortion through thedeblocking filter with the original picture. The SAO algorithm may addan offset, which is a difference from the original picture, to eachpixel in the residual block to which the deblocking filter was applied.Such post-processing filters may be applied to a reconstructed pictureor block.

The deblocking filter may be applied to pixels in several columns orrows included in blocks disposed on both sides of block boundaries. Itmay be preferable that the deblocking filter is applied to theboundaries of the encoded block, the prediction block, and the transformblock. Also, the deblocking filter may be applied limitedly to a blockhaving a size larger than or equal to a predetermined minimum size (forexample, 8×8 block).

Whether to apply a deblocking filter and a strength of the deblockingfilter may be determined based on properties of the block boundaries.The strength of the deblocking filtering may be chosen among thecandidates consisting of strong filtering, intermediate filtering, andweak filtering. When a block boundary corresponds to a boundary ofpartitioned unit, whether to apply a filter may be determined accordingto a flag indicating an application of an in-loop filter at the boundaryof the partitioned unit. The application of the deblocking filter invarious cases according to the present disclosure will be describedbelow.

The SAO may be applied based on a difference between the reconstructedpicture and the original picture. Types of the offset may include anedge offset and a band offset. One of those offsets may be selectedaccording to characteristics of the picture, and the filtering may beperformed for the selected offset. Meanwhile, information related to theoffset may be encoded in a unit of a block and, in particular, may beencoded using a predicted value thereof. In such a case, theoffset-related information may be adaptively encoded depending onwhether the predicted value is correct or not. The predicted value maybe offset information in adjacent blocks (for example, left, upper,upper left, or upper right block), and selection information about ablock from which the offset information is obtained may be generated.

A validity check may be performed when a group of candidates isconfigured. A candidate is included in the group of candidates in casethat the candidate is valid, but the validity check is passed to a nextcandidate when the candidate is invalid. A candidate may be regarded asbeing invalid also when a neighboring block is located outside thepicture, does not belong to a partitioned unit to which the currentblock belongs, or cannot be referenced as described below.

The encoded picture buffer 240 may store a block or picturereconstructed by the filter 235. The reconstructed block or picturestored in the encoded picture buffer 240 may be provided to thepredictor 200 performing the intra frame prediction or the inter frameprediction.

The entropy encoder 245 scans quantized transform coefficients accordingto a certain scanning pattern and encodes the quantized transformcoefficients using an entropy encoding scheme. The scanning pattern maybe one of various patterns such as a zigzag scan, a diagonal scan, and araster scan. In addition, the entropy encoder 245 may encode variousinformation including those received from the internal components of theencoder and output the encoded data as a bitstream.

FIG. 3 is a block diagram of a picture decoding device.

Referring to FIG. 3 , the picture decoding device 30 includes an entropydecoder 305, a predictor 310, an inverse quantizer 315, an inversetransformer 320, an adder/subtractor 325, a filter 330, and a decodedpicture buffer 335.

The predictor 310 may include an intra frame predictor module and aninter frame predictor.

First, a picture bitstream received by the picture encoding device 20may be transferred to the entropy decoder 305.

The entropy decoder 305 may decode the bitstream to obtain decoded dataincluding the quantized transform coefficients and encoding informationprovided by components of the encoder.

The predictor 310 may generate the prediction block based on the dataoutput by the entropy decoder 305. At this time, the predictor 310 mayconstruct a reference picture list by a default configuration schemebased on reference pictures stored in the decoded picture buffer 335.

The intra frame predictor may include a reference pixel constructor, areference pixel filter, a reference pixel interpolator, a predictionblock generator, and a prediction mode decoder. The inter framepredictor may include a reference picture constructor, a motioncompensator, and motion information decoder. Some components of thepredictor may perform the same process as those of the predictor in thepicture encoding device, and the other components of the predictor mayperform inverse operations of those of the predictor in the pictureencoding device.

The inverse quantizer 315 may inversely quantize the quantized transformcoefficients output by the entropy decoder 305.

The inverse transformer 320 may generate the residual block by applyingan inverse DCT, an inverse integer transform, or a similar inversetransform scheme to the transform coefficients.

The inverse quantizer 315 and the inverse transformer 320 may performinverse operations of those performed by the transformer 210 and thequantizer 215 of the picture encoding device 20 described above and maybe implemented in various methods. For example, the inverse quantizer315 and the inverse transformer 320 may share the functionalities withthe transformer 210 and the quantizer 215 to perform the inverse processand the inverse transform, or may perform inverse operations of thetransform and the quantization by use of the information about thetransform and the quantization processes received from the pictureencoding device 20 (e.g., the type and size of the transform, and thetype of the quantization).

The residual block generated after the inverse quantization and theinverse transform processes may be added to the prediction block outputby the predictor 310 to generate a reconstructed picture block. Theaddition may be performed by the adder/subtractor 325.

The filter 330 may apply a deblocking filter to the reconstructedpicture block as required to remove the blocking effect. The filter 330may additionally use other loop filters before or after the decodingprocess to enhance the picture quality.

The picture block resulted from the reconstruction and filteringprocesses may be stored in the decoded picture buffer 335.

A process of transforming color coordinate axes according to anembodiment of the present disclosure may be performed before the pictureencoding method described above or after the picture decoding methoddescribed above. Though the picture encoding and/or decoding methods wasdescribed above in terms of an embodiment of an MPEG compression scheme,the present disclosure is not limited thereto and may be used for a JPEGcompression scheme and other various picture encoding and/or decodingschemes.

FIG. 4 shows a sample picture.

A color picture acquired by a photographing device such as a camera isgenerally represented by pixel values in an RGB color space, and apicture which is to be displayed to a user after a decoding process maybe represented by pixel values in the RGB color space also.

The RGB color space refers to a system in which each pixel in a pictureis represented by three chromaticities of Red (R), Green (G), and Blue(B), and a color of a pixel may be represented by intensities of thethree chromaticity components.

Referring to FIG. 4 , the sample picture is a still picture in which 512pixels may be arranged in horizontal and vertical directions, and thusmay include a total of 262,144 (512×512) pixels. Each pixel may have R,G, and B values, and each of the R, G, and B value may have an integerranging from 0 to 255.

A picture encoding method using color coordinate axes transformationaccording to an embodiment of the present disclosure will be describedusing the sample picture with reference to FIGS. 5-7 .

FIG. 5 shows a pixel distribution of the sample picture in the RGB colorspace.

FIG. 5 shows a distribution of pixel values of the pixels in the samplepicture of FIG. 4 in a three-dimensional RGB color space, and therepresentation may be referred to as an RGB pixel distribution of thesample picture.

Referring to FIG. 5 , pixels of the sample picture are scattered in theRGB color space to some extent, but the variance of pixel values foreach color component may not be greatly different from the variances ofpixel values for the other color components. Here, the variance may bereferred to as a variation also.

The variance of pixel values for each color component may affect thecompression efficiency during a picture encoding. In other words, whenthere is little significant difference between the variances of pixelvalues for the color components, the compression efficiency may belowered due to a large spatial redundancy. Meanwhile, in case that thevariance of pixel values for a color component is large compared to theother color components, the picture may be encoded by allocating morebits to the color component having larger variance of pixel values whileallocating fewer bits to the color components so that the compressionefficiency may be increased due to a reduced spatial redundancy. Such ause of redundancies may be compared to: a sub sampling of Cb- andCr-components having relatively small variances in the JPEG and MPEGcompression; and performing the DCT to increase the variances of lowfrequency component coefficients and decrease the variances of highfrequency component coefficients for allocating more bits to the lowfrequency component coefficients.

The variance may be represented by E(X²)−{E(X)}² when the pixel valuedata is modeled as a random variable X and an expectation or astatistical average is expressed using a notation E{ }. However, thepresent disclosure is not limited thereto, and the variance may berepresented by any measurement indices coefficients that may indicatethe variation of data, e.g., E{|Xn|}−|E{X}|n where n is a natural numbergreater than or equal to 1.

In most of conventional picture encoding methods, pixel values of apicture are transformed from the RGB color space to the YCbCr colorspace to encode the picture with a high compression efficiency because asingle component, Y-component, has a higher variance than the othercomponents, details of which will be described below with reference toFIG. 6 .

FIG. 6 shows the pixel distribution of the sample picture in the YCbCrcolor space.

FIG. 6 may be a representation of pixel values for the sample pictureobtained by transforming the pixel distribution in the RGB color spaceof the sample picture shown in FIG. 5 into the pixel distribution in theYCbCr color space.

Conventional picture encoding methods such as JPEG and MPEG does notdirectly encode a picture represented in the RGB color space but encodesthe picture after transforming the pixel values of the picture from theRGB color space to the YCbCr color space.

The YCbCr color space refers to a system in which each pixel in apicture is represented by one luminance component (i.e., a luma sample)and two color difference components (i.e., chroma samples), and a colorof a pixel may be represented by intensities of the three components.

In case that the value of each component in the RGB color space isnormalized to a value between 0 and 1, each component in the YCbCr colorspace may be calculated from the components in the RGB color space asshown in Equation 1, and each component in the RGB color space may becalculated from the components in the YCbCr color space as shown inEquation 2.

$\begin{matrix}{\begin{bmatrix}Y \\C_{b} \\C_{r}\end{bmatrix} = {\begin{bmatrix}{{0.2}9900} & {{0.5}8700} & {{0.1}1400} \\{{- {0.1}}6874} & {{0.3}3126} & {{0.5}0000} \\{{0.5}0000} & {{- {0.4}}1869} & {{- {0.0}}8131}\end{bmatrix}\begin{bmatrix}R \\G \\B\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack \\{\begin{bmatrix}R \\G \\B\end{bmatrix} = {\begin{bmatrix}{{1.0}0000} & {{0.0}0000} & {{1.4}0200} \\{{1.0}0000} & {{- {0.3}}4414} & {{- {0.7}}1414} \\{{1.0}0000} & {{1.7}7200} & {{0.0}0000}\end{bmatrix}\begin{bmatrix}Y \\C_{b} \\C_{r}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

The components in the RGB color space for the samples shown in FIG. 5may be transformed into components in the YCbCr color space according tothe Equation 1, and FIG. 6 is a representation of the samples in athree-dimensional YCbCr color space. The representation of FIG. 6 may bereferred to as a YCbCr pixel distribution of the sample picture.

Referring to FIG. 6 , for the pixels of the sample picture representedin the YCbCr color space, the luminance component (Y-component) may bemore scattered and have a higher variance than the blue-differencechroma component (Cb-component) and the red-difference chroma component(Cr-component), and thus more bits may be concentrated in theY-component than the Cb-component and the Cr-component during thepicture encoding.

In other words, the transformation into the YCbCr color space allows theconcentration of bits on the Y-component, which may enhance thecompression efficiency during the picture encoding as described above.

FIG. 7 is a graph showing a pixel distribution of the sample picture ina color coordinate system obtained by a transformation of colorcoordinate axes according to an embodiment of the present disclosure.

A picture encoding method according to an embodiment of the presentdisclosure may further enhance the compression efficiency than the useof the YCbCr pixel distribution shown in FIG. 6 through a colorcoordinate axes transformation. In detail, the picture encoding methodaccording to an embodiment of the present disclosure may generate a newYCbCr color coordinate system from the existing YCbCr color coordinatesystem through the coordinate axes transformation. However, the presentdisclosure is not limited to a method of transforming the YCbCr colorcoordinate system to a new color coordinate system through the colorcoordinate axes transformation, but may be applied to all colorcoordinate systems and all kinds of the coordinate axes transformationwhich can enhance the compression efficiency.

Here, the existing YCbCr color coordinate system may be represented by aYCbCr color coordinate system or a first color coordinate system, and anew color coordinate system may be represented by a Y′Cb′Cr′ colorcoordinate system or a second color coordinate system.

The process of transforming the color coordinate axes according to anembodiment of the present disclosure may be generally divided into threeoperations. Each operation will now be described in detail.

In a first operation, coordinate axes of the first coordinate system isrotated such that the variance of the Y-component is maximized. At thistime, a coordinate axis of the Y-component may be fixed at an origin,and a coordinate axis of the Y-component, a coordinate axis of theCb-component, and a coordinate axis of the Cr-component may bemaintained perpendicular to each other. Through the rotation, thecoordinate axis of the Y-component as well as the coordinate axis of theCb-component and the coordinate axis of the Cr-component of the firstcolor coordinate system may be transformed. A color coordinate systemgenerated through the rotation may be referred to as an intermediatecolor coordinate system.

The first operation may be performed after moving the origin of thefirst color coordinate system to a central point of the pixeldistribution such that an average of each component becomes zero.

In a second operation, a variance of the Cb-component is compared with avariance of the Cr-component in the intermediate color coordinatesystem, and a component having a larger variance is determined.

In a third operation, coordinate axes of the intermediate colorcoordinate system is rotated such that the variance of the componentdetermined in the second operation is maximized. At this time, thecoordinate axis of the Y-component of the intermediate color coordinatesystem may be fixed, and the coordinate axis of the Cb-component and thecoordinate axis of the Cr-component may be maintained perpendicular toeach other. A resultant color coordinate system may be a final colorcoordinate system generated through the transformation of the colorcoordinate axes and may be referred to as the second color coordinatesystem. Each component of the second color coordinate system may bedenoted by a Y′-component, a Cb′-component, and a Cr′-component.

The process for transforming the color coordinate axes according to anembodiment of the present disclosure may be performed afterdown-sampling the components of the existing YCbCr color coordinatesystem in the three-dimensional space in order to reduce a complexity ora calculation burden.

FIG. 7 is a graph showing a pixel distribution of the sample picture inthe Y′Cb′Cr′ color coordinate system (i.e., the second color coordinatesystem) obtained by the transformation of the color coordinate axes onthe pixel distribution of the sample picture in the YCbCr colorcoordinate system (i.e., the first color coordinate system).

Referring to FIG. 7 , it can be seen that the variance of theY′-component in the second color coordinate system has a larger variancethan the Y-component in the first color coordinate system, and theCb′-component and the Cr′-component in the second color coordinatesystem have a smaller variance than those in the first color coordinatesystem. In other words, the second color coordinate system has acomponent which became more prominent than the first color coordinatesystem. Thus, more bits may be concentrated in the prominent component,or the compression efficiency may be enhanced.

The picture encoding method according to an embodiment of the presentdisclosure may transform the color coordinate axes through theabove-described process. The information about the coordinate axestransformation may be included in a compressed picture file in a form ofoverhead information for each picture. Here, the data regarding theoverhead information has a very small size compared to the size of thecompressed picture file and thus may be ignorable in the total amount ofdata.

The size of the data regarding the coordinate axes transformationinformation may be reduced further by referring to a previous picture ora previous block. For example, in case that a still picture ispartitioned into blocks having a size 8×8 pixels and the blocks aresequentially compressed in a direction from a upper left block to thelower right block during a JPEG picture compression, the transformationof the color coordinate axes may be performed in a unit of the block andthe coordinate axis transformation information for each block may beincluded in the form of the overhead information according to anembodiment of the present disclosure. Here, the coordinate axestransformation information may be included in the form of the overheadinformation for each block, but the coordinate axes transformationinformation for a current block may be obtained by performing colorcoordinate axes transformation based on previous blocks which arecompressed already or some pixels in the previous blocks. The coordinateaxes transformation information for the current block obtained as suchmay be referred to as predicted coordinate axes transformationinformation.

Thus, according to the picture encoding method, the transformation ofthe coordinate axes is performed for each of the blocks into which apicture is partitioned, and the coordinate axes transformationinformation may exists for each block. Alternatively, the coordinateaxes transformation information may be predicted based on the previousblocks or some pixels of the previous blocks, and there may existpredicted coordinate axes transformation information. In a latter case,the overhead information for each block may include informationindicating whether the coordinate axes transformation information ispredicted or actual.

Here, the information indicating whether the coordinate axestransformation information is predicted or actual may further includeinformation about the previous blocks or some pixels of the previousblocks when the transformation of the coordinate axes is performed forthe current block based on the previous blocks or some pixels in theprevious blocks and there exists predicted coordinate axestransformation information.

The picture encoding method according to an embodiment of the presentdisclosure may or may not perform the color coordinate axestransformation for each of the blocks into which a picture ispartitioned, and the overhead information for each block may include thetransformation execution information indicating whether the colorcoordinate axes were transformed or not.

Here, the transformation execution information may be assigned with onebit as an indicator or a flag, The transformation of the coordinate axesmay be omitted when the transformation results in little enhancement inthe compression efficiency or when the increase in the compressionefficiency is less than a preset value.

FIG. 8 is a graph showing an enhanced compression efficiency when thepicture encoding is performed after the transformation of colorcoordinate axes according to an embodiment of the present disclosure.

Referring to FIG. 8 , a first case (denoted by ‘case 1’) denotes a casewhere the sample picture of FIG. 4 is compressed by the typical JPEGscheme, and a second case (denoted by ‘case 2’) is a case where thesample picture encoded by the JPEG scheme after the color coordinateaxes are transformed according to an embodiment of the presentdisclosure.

Here, the transformation execution information may be included in theoverhead information for each block and may include a 1-bit indicator.In addition, the transformation execution information may include anindicator. The indicator has a value of zero when the transformation ofthe color coordinate axes according to an embodiment is performed whilehaving a value of one when the transformation of the color coordinateaxes is not performed.

Also, the prediction information may be included in the overheadinformation for each block, and the transformation execution informationmay be included only in the overhead information of a block for whichthe transformation of the color coordinate axes is performed. Accordingto an embodiment of the present disclosure, when the transformation ofthe color coordinate axes is performed for the current block based onthe previous blocks or some pixels of the previous blocks according tothe prediction information, the pixel distribution information of thecurrent block may be predicted and acquired based on all the pixels infour neighboring blocks, i.e., the left, upper left, upper, and upperright blocks of the current block, the transformation of the colorcoordinate axes is performed for the current block based on the acquiredpixel distribution information of the current block.

Referring to FIG. 8 , the compression efficiencies of the first andsecond cases in the picture encoding and decoding may be compared witheach other by use of a peak signal-to-noise ratio (PSNR) and a bit perpixel (bpp). It can be seen that the second case indicating the pictureencoding method using the color coordinate axes transformation accordingto an embodiment of the present disclosure has a superior compressionefficiency than the first case. Here, it should be noted that the PSNRis indicated in units of decibels (dB) based on logarithm.

Though the above description was focused on the JPEG compression with ablock size of 8×8 pixels, the present disclosure may be applied to anMPEG compression. In such a case, the block size may not be fixed at 8×8pixels and may be variable.

In other words, when the picture encoding method using the colorcoordinate axes transformation according to an embodiment of the presentdisclosure is applied to the MPEG compression, the encoding process maydiffer from the process for the JPEG compression in configurations forpredicting the current block, by the intra frame prediction or a spatialprediction, using the neighboring blocks and pixels in apreviously-compressed intra frame, predicting the current block, by theinter frame prediction or a temporal prediction, using the neighboringblocks and pixels in the inter frames, calculating a prediction error,and performing the compression.

FIG. 9 is a block diagram of a picture encoding device according to anembodiment of the present disclosure.

Referring to FIG. 9 , the picture encoding device 900 according to anembodiment of the present disclosure may include at least one processor910, a memory 920, and a storage 930.

The picture encoding device 900 according to an embodiment of thepresent disclosure may be disposed before or after a general pictureencoding and decoding device or may be mounted in the general pictureencoding and decoding device to perform necessary processes before orafter the picture encoding and decoding operation, but the presentdisclosure is not limited thereto.

The processor 910 may execute program commands or instructions stored inthe memory 920 and/or the storage 930. The processor 910 may be acentral processing unit (CPU), a graphics processing unit (GPU), or adedicated processor suitable for performing the methods of the presentdisclosure. The memory 920 and the storage 930 may include a volatilestorage medium and/or a non-volatile storage medium. For example, thememory 920 may include a read-only memory (ROM) and/or a random accessmemory (RAM).

The memory 920 may store at least one instruction to be executed by theprocessor 910. The at least one instruction may include an instructionfor acquiring pixel distribution information for the picture in thefirst color coordinate system, an instruction for determining a firstcomponent having the largest variance among the components in the firstcolor coordinate system on the basis of the pixel distributioninformation, an instruction for rotating the coordinate axes of thefirst color coordinate system around the origin such that the varianceof the first component is maximized to acquire an intermediate colorcoordinate system, an instruction for acquiring a second componenthaving the larger variance between the remaining components of theintermediate color coordinate system other than the first component, aninstruction for rotating the coordinate axes of the intermediate colorcoordinate system around the origin such that the variance of the secondcomponent is maximized to acquire the second color coordinate system,and an instruction for encoding the picture on the basis of the pixeldistribution information in the second color coordinate system.

In addition, the at least one instruction may include an instruction forgenerating coordinate axes transformation information for the picture onthe basis of a difference between the pixel distribution information inthe first color coordinate system and the pixel distribution informationin the second color coordinate system or an instruction for generatingthe overhead information for the picture on the basis of the coordinateaxes transformation information for the picture.

In addition, the at least one instruction may include at least one of:an instruction for determining whether to perform the transformation ofthe coordinate axes for the picture on the basis of a difference betweenthe pixel distribution information in the first color coordinate systemand the pixel distribution information in the second color coordinatesystem, an instruction for generating the transformation executioninformation depending on whether to perform the transformation of thecoordinate axes for the picture, and an instruction for generating theoverhead information for the picture based on the transformationexecution information.

In addition, the at least one instruction may include at least one of:an instruction for acquiring the pixel distribution information in thefirst color coordinate system for the picture on the basis of the pixeldistribution information in the first color coordinate system for atleast one different picture and an instruction for generating theoverhead information for the picture on the basis of the at least onedifferent picture.

Here, the first color coordinate system may include a YCbCr colorcoordinate system, and the first component may include a Y-component.

FIG. 10 is a flowchart showing a picture encoding method according to anembodiment of the present disclosure.

Referring to FIG. 10 , the picture encoding device according to anembodiment of the present disclosure may first acquire pixeldistribution information for a picture in the existing YCbCr colorcoordinate system (S1010), and acquire an intermediate color coordinatesystem by rotating a coordinate axis of the Y-component of the existingYCbCr color coordinate system such that the variance of the Y-componentis maximized (S1020). Here, the coordinate axes of the existing YCbCrcolor coordinate system are rotated by rotating the coordinate axis ofthe Y-component because the Y-component typically shows the largestvariance among the Y-component, the Cb-component, and the Cr-componentin the YCbCr color coordinate system, but the axis to be rotated is notlimited to the Y-component axis. The rotation of the coordinate axes ofthe existing YCbCr color coordinate system may be carried out whilemaintaining the coordinate axis of the Y-component, the coordinate axisof the Cb-component, and the coordinate axis of the Cr-componentperpendicular to each other. Before the rotation, the origin of thecoordinate system may be displaced based on the pixel distribution,i.e., such that the average of each component in the pixel distributionmay be zero.

Subsequently, the picture encoding device may determine a componentshowing a larger variance between the Cb-component and the Cr-componentin the intermediate color coordinate system (S1030) and acquire atransformed color coordinate system by rotating coordinate axes of theintermediate color coordinate system such that the variance of thedetermined component is maximized (S1040). Here, the rotation of thecoordinate axes of the intermediate color coordinate system may becarried out around the origin of the intermediate color coordinatesystem while maintaining the coordinate axis of the Cb-component and thecoordinate axis of the Cr-component perpendicular to each other. Thetransformed color coordinate system may be a final color coordinatesystem.

The picture encoding device may generate coordinate axis transformationinformation on the basis of a difference between the pixel distributioninformation in the existing color coordinate system and the pixeldistribution information in the transformed color coordinate system(S1050) and may add the generated coordinate axis transformationinformation to additional information for the picture (S1060). Then, thepicture encoding device may encode the picture on the basis of the pixeldistribution information in the transformed color coordinate system(S1070). Afterwards, the picture decoding device may inversely transformthe pixel distribution information in the transformed coordinate systeminto the pixel distribution information in the existing color coordinatesystem on the basis of the coordinate axis transformation information.

In other words, while a conventional picture encoding device typicallyacquires and encodes pixel values information in the YCbCr colorcoordinate system, the picture encoding device according to presentdisclosure may acquire and encode pixel values information in theY′Cb′Cr′ color coordinate system. The picture decoding device mayperform an inverse operations of those described above, so thatinformation acquired by the decoding operation may be the information inthe Y′Cb′Cr′ color coordinate system and such information may beinversely transformed into the YCbCr color coordinate system.

The operations of the methods according to an embodiments may beembodied as computer-readable programs or codes on a computer-readablerecording medium. The computer-readable recording medium is any datastorage device that can store data that can thereafter be read by acomputer system. In addition, the computer-readable recording medium mayalso be distributed over network-coupled computer systems so that thecomputer-readable program or code is stored and executed in adistributed fashion.

In addition, the computer-readable recording medium may include ahardware device specially constructed to store and execute a programinstruction, for example, a ROM, a RAM, and a flash memory. The programinstruction may include a high-level language code executable by acomputer through an interpreter in addition to a machine language codemade by a compiler.

Some of the aspects of the present disclosure have been described in thecontext of the device but may represent the description of a methodcorresponding thereto, and a block or a device corresponds to anoperation of a method or a feature thereof. Similarly, some of theaspects having been described in the context of the method may also berepresented by a block or items corresponding to the method or a featureof a device corresponding to the method. Some or all of the operationsof the method may be performed, for example, by (or using) the hardwaredevice, such as a microprocessor, a programmable computer, or anelectronic circuit. In some embodiments, one or more of the mostimportant operations of the method may be performed by such a device.

In some embodiments, a programmable logic device (for example, a fieldprogrammable gate array) may be used to perform some or all of the abovedescribed functions of the methods. In some embodiments, a fieldprogrammable gate array may operate together with a microprocessor toperform one of the above described methods. In an implementation, themethods may be performed by any hardware device.

Although the embodiments of the present disclosure have been describedin detail, it should be understood that various substitutions,additions, and modifications are possible without departing from thescope and spirit of the present disclosure, and the scope of the presentdisclosure is limited by the claims and the equivalents thereof

What is claimed is:
 1. A picture encoding method, comprising: acquiringpixel distribution information for a picture in a first color coordinatesystem; determining a first color component showing a largest variancein pixel values among color components of pixels of the picture in thefirst color coordinate system on the basis of the pixel distributioninformation; rotating coordinate axes of the first color coordinatesystem around an origin while maintaining a coordinate axis of one ofthe color components, excluding the first color component, and acoordinate axis of the other one of the color components perpendicularto each other such that the variance of the first color component ismaximized to acquire an intermediate color coordinate system;determining a second color component showing a larger variance in pixelvalues between color components, excluding the first color component, ofthe intermediate color coordinate system; rotating coordinate axes ofthe intermediate color coordinate system around the origin such that thevariance of the second color component is maximized to acquire a secondcolor coordinate system; and encoding the picture on the basis of pixeldistribution information in the second color coordinate system.
 2. Themethod of claim 1, further comprising: generating coordinate axestransformation information for the picture on the basis of a differencebetween the pixel distribution information in the first color coordinatesystem and the pixel distribution information in the second colorcoordinate system.
 3. The method of claim 2, further comprising:generating additional information about the picture on the basis of thecoordinate axes transformation information for the picture.
 4. Themethod of claim 1, further comprising: determining whether to transformcoordinate axes for the picture on the basis of a difference between thepixel distribution information on the first color coordinate system andthe pixel distribution information on the second color coordinatesystem.
 5. The method of claim 4, further comprising: generatinginformation of whether the coordinate axes are transformed depending onwhether the coordinate axes are transformed; and generating additionalinformation about the picture based on the information of whether thecoordinate axes are transformed.
 6. The method of claim 1, whereinacquiring the pixel distribution information for the picture in thefirst color coordinate system comprises: acquiring the pixeldistribution information for the picture in the first color coordinatesystem on the basis of pixel distribution information for at least oneanother picture in the first color coordinate system.
 7. The method ofclaim 6, further comprising: generating additional information about thepicture on the basis of information about the at least one anotherpicture.
 8. The method of claim 1, wherein the first color coordinatesystem includes a YCbCr color coordinate system, and the first colorcomponent includes a Y-component.
 9. A picture encoding device,comprising: a processor; and a memory storing at least one instructionto be executed by the processor, wherein the at least one instructionwhen executed by the processor causes the processor to: acquire pixeldistribution information for a picture in a first color coordinatesystem; determine a first color component showing a largest variance inpixel values among color components of pixels in the first colorcoordinate system on the basis of the pixel distribution information;rotate coordinate axes of the first color coordinate system around anorigin such that the variance of the first color component is maximizedto acquire an intermediate color coordinate system; determine a secondcolor component showing a larger variance in pixel values between colorcomponents, excluding the first color component, of the intermediatecolor coordinate system; rotate coordinate axes of the intermediatecolor coordinate system around the origin while maintaining a coordinateaxis of one of the color components, excluding the first colorcomponent, and a coordinate axis of the other one of the colorcomponents perpendicular to each other such that the variance of thesecond color component is maximized to acquire a second color coordinatesystem; and encode the picture on the basis of pixel distributioninformation in the second color coordinate system.
 10. The device ofclaim 9, wherein the at least one instruction when executed by theprocessor further causes the processor to: generate coordinate axestransformation information for the picture on the basis of a differencebetween the pixel distribution information in the first color coordinatesystem and the pixel distribution information in the second colorcoordinate system.
 11. The device of claim 10, wherein the at least oneinstruction when executed by the processor further causes the processorto: generate additional information about the picture on the basis ofthe coordinate axes transformation information for the picture.
 12. Thedevice of claim 9, wherein the at least one instruction when executed bythe processor further causes the processor to: determine whether totransform coordinate axes for the picture on the basis of a differencebetween the pixel distribution information on the first color coordinatesystem and the pixel distribution information on the second colorcoordinate system.
 13. The device of claim 12, wherein the at least oneinstruction when executed by the processor further causes the processorto: generate information of whether the coordinate axes are transformeddepending on whether the coordinate axes are transformed; and generateadditional information about the picture based on the information ofwhether the coordinate axes are transformed.
 14. The device of claim 9,wherein the at least one instruction when executed by the processorfurther causes the processor to: acquire the pixel distributioninformation for the picture in the first color coordinate system on thebasis of pixel distribution information for at least one another picturein the first color coordinate system.
 15. The device of claim 14,wherein the at least one instruction when executed by the processorfurther causes the processor to: generate additional information aboutthe picture on the basis of information about the at least one anotherpicture.
 16. The device of claim 9, wherein the first color coordinatesystem includes a YCbCr color coordinate system, and the first colorcomponent includes a Y-component.